Almost six months after the companies started talking, WPP and Microsoft have reopened talks that could have the software company unloading Avenue A/Razorfish. But the question is whether Microsoft could ever get anyone to buy the digital ad agency for the price at which it needs to sell it.

What Microsoft paid for the agency and what any holding company would shell out are vastly different figures — although WPP holds an edge over other holding companies because it has assets Microsoft might be interested in, namely the ad-serving technology bit of 24/7 Real Media.

Here’s how a deal could unfold, according to people familiar with the discussions: Microsoft unloads the agency in exchange for a WPP package that includes 24/7′s Open AdStream publisher ad-serving tool plus cash. While Avenue A’s price would be higher than most agency deals, very few interactive agencies with that kind of scale are available for acquisition.

Avenue A|Razorfish is an ad agency specializing in interactive media and is an uncomfortable fit for Microsoft which wants to make money from all advertisers and publishers without questions of favoritism about a house agency. Similarly, WPP Group is a holding company for advertising agencies and the Open AdStream ad serving tool doesn’t fit either.

Klaassen’s thesis is that since Microsoft overpaid for aQuantitive, it is going to be hard to find someone to pay for Avenue A|Razorfish anywhere near what Microsoft paid. That’s why a swap with WPP Group is a good fit since it is similarly believed to have overpaid for 24/7 Real Media and they could obfuscate the real value with a swap. I’m dubious as to how important that reason really is since acquisitions often result in a grab bag of extra pieces that are sold off at "bargain sale" valuations. Of course, since Avenue A|Razorfish accounted for 60% of aQuantive’s revenues, it is a trifle hard to see which is the tail and which is the dog.

In any case, beyond face-saving reasons, it seems like a good match. WPP specializes in owning ad agencies and Microsoft needs an ad serving service in their eternal quest to keep up with Google which is busy widening the beta of their competing (and free) Ad Manager service.